I specifically mentioned torrenting, please re-read. There is no reason to have all the network I/O of your computer go through an extra hop just for one application (e.g. a torrent client) when that application can be configured to use a proxy instead. It increases the network latency, complexity and attack surface of your system.

If the main use case is torrents,

I think the reason why the method you mention isn't commonly used is that it is complicated to understand/set up and hard to verify.

I've seen more advanced users encapsulate everything in a VM so that non-VPN traffic can be blocked globally by the OS.